1. Field of the Invention
The present invention relates generally to a surface acoustic wave sensor and an interrogator apparatus for wireless communication with the surface acoustic wave sensor and to a method for distinguishing between multiple sensors.
2. Description of the Related Art
The simultaneous interrogation of uniquely identifiable wireless sensors is recognized as a highly desirable goal. To facilitate achieving this goal, active sensor nodes and passive-acting sensor nodes containing active components often contain memory that stores a code to identify each sensor. The interrogating system can interact with these sensors using a “handshake” process that instructs the sensor when to take a measurement and return its information. Thus, sensors can be instructed to delay their return signal by a specified delay in order to reduce collisions between signals from multiple sensors, or they can be instructed to re-send the measurement, among other options. This level of bi-directional interaction, with the sensor modifying its response to interrogation based on specific instructions, is only possible in systems where the sensors contain active components. In a wireless sensor system utilizing purely passive sensors such as acoustic wave (or surface acoustic wave—SAW) sensors, the sensor acts as a passive reflector for the radio frequency (RF) interrogation signal. When a broadcast interrogation signal is being used (as compared to broadcasting a coded signal specific to one of the sensors), all of the SAW sensors within the range of a particular interrogator will reflect back sensor responses, each containing information on the sensor identification and the measured (or sensed) parameter. Since there is no way to select a single sensor in such a broadcast system, these response signals will arrive at the receiver at slightly varying times based on their physical locations due to the small RF signal propagation delay. In order to resolve the individual sensor signal responses, it is necessary to achieve discrimination between sensor responses, which has historically been achieved for SAW sensors using sensor coding.
Well known wireless coded SAW sensors include single frequency “tag” sensors such as the multiple tap reflective delay line temperature sensor shown in 1981 by Vaughan (U.S. Pat. No. 4,399,441), and the orthogonal frequency coded (OFC) SAW sensor developed recently by Malocha. Other RFID tags using SAW also use such encoding techniques, including various coding approaches (on-off keying, phase coding, etc.). These devices incorporate a code in the reflector structures on the SAW surface to produce reflections that occur at specific times. In the traditional single frequency reflective tag approach, there is generally a “start” reflector and a “stop” reflector to indicate when the interrogator can start to read the response, and when the response is complete. Between these two reflectors are multiple reflectors that are either present or absent (or in other variations the positions are modified to alter the phase of the reflection) to create a code, along with error checking bits. The start bit in this case effectively “turns on” the signal processing in the interrogator to initiate code identification. Such single frequency tags have been used successfully in applications that can utilize scanning of a single tag at a time, at relatively short range, such as monitoring identification of livestock or automobile toll booths. Such approaches are not well suited to use in a system where multiple sensors would respond to interrogation in a given time window, as the signals cannot be adequately resolved from one another.
OFC (orthogonal frequency coded) sensors attempted to address this issue by providing codes within the sensors that are inherently orthogonal. The mathematical relationship between codes used in a sensor system is such that each code will correlate well with itself, producing a clear correlation peak (or in the case of differential delay line sensors, producing a pair of correlation peaks), while interacting with different codes to produce cross correlations that are much lower and without clear peaks. In theory this should allow for clear sensor discrimination. However, in practice this approach encounters significant problems. Specifically, the orthogonality of the codes is only maintained if they are operating in a synchronous system. Because of the random RF propagation delays experienced by signals from each sensor, and due to changes in delay due to the sensed parameter, even ideal orthogonal codes will necessarily produce non-orthogonal interfering signals. This issue, which has been widely recognized in the realm of CDMA (code division multiple access) communication systems, makes the use of orthogonal codes alone as a basis for passive sensor discrimination ineffective.
It is well known in the wireless communication industry that there are several possible approaches to achieve multi-user access to shared bandwidth. The ability of each approach to provide robust, reliable demodulation of multiple randomly occurring signals without interchannel interference is a measure of the efficacy of the approach. Well known approaches include Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), and Frequency Division Multiple Access (FDMA). CDMA systems assign different codes (or signature waveforms) to each user, and the interrogation system is responsible for identifying the sensor from the collective responses received. TDMA systems utilize signature waveforms that do not overlap in the time domain—one user utilizes the channel, then another, and so forth. FDMA systems separate users in the frequency domain by allocating specific bandwidth to each user, so that signature waveforms do not overlap in the frequency domain. Strictly speaking, TDMA and FDMA systems do not need to utilize coded waveforms unless the resource (time or frequency slot) allocation is dynamic, although the inclusion of such codes increases system capacity.
When reviewing wireless SAW sensor systems developed to date, it is clear that in those systems where multiple simultaneous sensor interrogation has been attempted, one of these standard approaches to multiple access has been utilized. For example, OFC sensors used a CDMA approach, while SAW resonator sensor systems with frequency diversity utilize FDMA. Not considered here are sensor systems that broadcast coded signals to interrogate a specific sensor, but rather systems that interrogate a group of sensors simultaneously and interpret the resulting sensor responses. It is clear that these techniques can be used to advantage when combined in ways that have not previously been demonstrated.
These earlier devices relate to the combination of CDMA and TDMA techniques in SAW sensor devices and systems, and the advantages obtained therefrom. In all prior coded SAW sensors, there has generally been a “pedestal” delay, or a nominal delay due to SAW propagation from the launching transducer to the first reflective tap and back, or to the first output transducer, etc. The measurements made by these sensors utilized the differential delays between this initial delayed response and subsequent responses—either in differential delay line sensors, or tag responses where all subsequent responses are referenced to the “start” bit. The pedestal delay is chosen to be any convenient delay, and this acoustic delay, augmented by the RF propagation delay, has been a noncritical value in sensor and system design. In fact, the slow acoustic propagation delay for this first response signal has been recognized as one of the major benefits of using acoustic wave sensor devices—i.e. RF multipath reflections will have time to decay away before the first acoustic response returns. But other than its utility for avoiding multipath, this nominal pedestal delay has been treated by prior sensor developers as non-critical.
U.S. Pat. Nos. 7,100,451, 7,268,662 B2, and 7,434,99 B2 have taught that two time integrating correlator channels can operate on adjacent frequency bands and determine the relative amounts of returned radio frequency (RF) energy in each frequency band. This ratio, in turn, can be used as a measurand to indicate some property to be sensed, e.g., temperature, pressure, chemical concentration, etc.